The present invention relates to a control system for a plant, and more particularly to a control system for controlling a plant with a sliding mode controller based on a sliding mode control theory which is one of robust control theories.
One known control system based on a sliding mode control theory is disclosed in Japanese Patent Laid-open No. Hei 9-274504, for example. The publication proposes a method of setting a hyperplane in the sliding mode control theory according to the convergence state of a controlled state quantity. According to the proposed method, the convergence response and convergence stability of the sliding mode control is improved.
In the above publication, it is shown that the setting of the hyperplane is changed according to the convergence state of the controlled state quantity so that the control response speed becomes higher. However, in the case that the controlled object which is controlled by the sliding mode controller has non-linear characteristics, good controllability cannot necessarily be obtained even if the control response speed is changed according to the convergence state of the control.
It is therefore an object of the present invention to provide a control system for a plant, which can more suitably set the control response speed of the sliding mode controller, to thereby maintain good controllability when the dynamic characteristics of the plant changes.
To achieve the above object, the present invention provides a control system for a plant, including a sliding mode controller (21) for controlling the plant with a sliding mode control so that an output of said plant may coincide with a control target value. The sliding mode controller (21) controls the plant using a switching function ("sgr") defined as a linear function of a deviation (e(k)) between the output (DTH) of the plant and the control target value (DTHR), and changes the switching function ("sgr") according to at least one of a parameter (DTH) indicative of the output of the plant and a parameter (DDTHR) indicative of an amount of change in the output of the plant.
With this configuration, the plant is controlled using the switching function defined as a linear function of the deviation between an output (DTH) of the plant and the control target value, and the switching function is changed according to at least one of a parameter (DTH) indicative of the output of the plant and a parameter indicative of an amount of change in the output of the plant. Accordingly, by appropriately changing the switching function according to the output (DTH) of the plant and/or the amount of change in the output (DTH) of the plant, the control response speed can be set to a value which is suitable for the output of the plant and/or the amount of change in the output of the plant, to thereby make it possible to maintain good controllability when the dynamic characteristics of the plant changes depending on the output of the plant. If the plant is a throttle valve actuating device, energizing forces exerted on the throttle valve change abruptly in the vicinity of a predetermined opening (THDEF). That is, the dynamic characteristic of the throttle valve actuating device changes greatly in the vicinity of the predetermined opening.
Preferably, the control input (Usl) applied from the sliding mode controller to the plant includes an adaptive law input (Uadp).
With this configuration, the control input applied to the plant includes the adaptive law input. By applying the adaptive law input, better controllability can be obtained even in the presence of disturbance and/or the modeling error (a difference between the characteristics of the actual plant and the characteristics of the controlled object model).
Specifically, the plant includes a throttle valve actuating device (10) having a throttle valve (3) of an internal combustion engine (1) and actuating means (6) for actuating the throttle valve (3), and throttle valve opening detecting means (8) for detecting an opening of the throttle valve as an output of the plant is mounted on the throttle valve actuating device (10). The sliding mode controller (21) calculates a parameter for determining a control input to be applied to the throttle valve actuating device (10) to make the detected opening (TH) of the throttle valve coincide with a target opening (THR).
In a throttle valve opening control system where an opening of the throttle valve which is actuated by a motor is controlled with the PID (Proportional, Integral, and Differential) control, it is conventionally known that the control constants of the PID control are set according to an operating condition of the vehicle (Japanese Patent Laid-open No. Hei 8-261050). However, according to the PID control, it is difficult to obtain both of good controllability and good control stability in such a case that the controlled object has a non-linear characteristics.
According to the above-described configuration of the present invention, the throttle valve opening is controlled to the target opening with the sliding mode control having good robustness. Therefore, both of good controllability of the throttle valve opening to the target opening and good control stability can be obtained even if the controlled object has a non-linear characteristics.
Preferably the throttle valve actuating device includes first energizing means (4) for energizing the throttle valve in a closing direction and second energizing means (5) for energizing the throttle valve in an opening direction, and maintains the throttle valve opening at a predetermined opening (THDEF) by the first and second energizing means (4, 5) when the throttle valve (3) is not actuated by the actuating means (6). The sliding mode controller changes the switching function ("sgr") so that a control response speed may increase, when the detected throttle valve opening (TH) is in the vicinity of the predetermined throttle valve opening (THDEF). The control response speed is a speed at which the throttle valve opening (TH) converges on the target opening (DTHR).
The dynamic characteristic of the throttle valve actuating device greatly changes when the throttle valve opening is in the vicinity of the predetermined throttle valve opening (THDEF). Therefore, according to the control system disclosed in the publication No. Hei 8-261050, it is difficult to obtain good controllability.
According to the above-described configuration of the present invention, the switching function is changed so that the control response speed may increase, when the detected throttle valve opening (TH) is in the vicinity of the predetermined throttle valve opening (THDEF). This makes it possible to maintain good controllability when the throttle valve opening is in the vicinity of the predetermined throttle valve opening.
Preferably, the sliding mode controller changes the switching function ("sgr") so that a control response speed may decrease, when the detected throttle valve opening (TH) is in the vicinity of a fully closed opening and/or a fully open opening.
When the PID control is employed for controlling the throttle valve opening, like the control system shown in the above publication No. Hei 8-261050, it is necessary to set the control constants to small values in order to avoid that the throttle valve collides in a high speed with the stoppers arranged at the fully closed position and the fully open position. Accordingly, sufficient control response speed cannot be obtained although the high speed response is necessary for controlling the throttle valve opening.
According to the above-described configuration of the present invention, the switching function is changed so that the control response speed may decrease, when the detected throttle valve opening (TH) is in the vicinity of a fully closed opening and/or a fully open opening. Therefore, it is possible to prevent the throttle valve from colliding in a high speed with the stopper arranged at the fully closed position and/or/or the fully open position. In addition, when the throttle valve opening is not in the vicinity of a fully closed opening or a fully open opening, sufficient control response speed can be obtained.